Permanent magnet motors have conventionally been used as power units for home appliances such as wash machines and clothes dryers, hybrid cars, electric trains, elevators, and the like. When driving a permanent magnet motor at variable speeds, however, the induced voltage therein increases in proportion to the rotational speed, since the permanent magnet has a fixed magnetic flux. The driving becomes hard at such a high rotational speed that the induced voltage is at the power-supply voltage or higher. Therefore, in a middle/high speed range or under light load, it has been necessary for the conventional permanent magnet motors to perform flux-weakening control for canceling out the magnetic flux of the permanent magnet with a magnetic flux caused by an armature current, which lowers the efficiency of the motors.
For solving such problems, variable-magnetic-flux motors using a magnet (variable-magnetic-force magnet) whose magnetic force reversibly changes under action of an external magnetic field have been developed in recent years. By lowering the magnetic force of the variable-magnetic-force magnet in the middle/high speed range or under light load, the variable-magnetic-flux motors can inhibit their efficiency from decreasing as in the conventional motors.